1. Field of the Invention
The present invention relates to a catalytic combustion element and a method of causing catalytic combustion.
2. Discussion of the Related Art
In Japanese Patent Laid-Open No. Hei 3-140705, there is disclosed a conventionally employed catalytic combustion element, which will be described with reference to FIG. 9. When a valve 21 is closed, a fan 22 is driven to supply air from a tubular passage 23a of an air supply tube 23, and fuel is supplied through a fuel supply tube 24. The air and fuel thus supplied collide with a rectifying plate 26 arranged inside a mixing chamber 25, are mixed uniformly with each other therein and are blown out from flame holes 27a formed in a combustion plate 27. Then the air-fuel mixture is ignited by an ignition unit 28 so that flames 29 are generated. The flames 29 heat a catalyst 30 which is arranged at the left end of a combustion cylinder shown in the figure. The temperature of the catalyst 30 is detected by a temperature sensor 31. When a temperature at which catalytic combustion can occur is reached, the valve 21 is opened to supply air from a tubular passage 23b. Then the flames 29 formed at the combustion plate 27 are blown off and transferred to the catalyst 30. Since the catalyst 30 has already reached a catalytic activation temperature, catalytic combustion is started immediately. The catalyst 30 can be employed in a variety of heating burners.
In Japanese Patent Laid-Open No. Hei 5-79613, there is disclosed another combustion apparatus which will be described with reference to FIG. 10. In this apparatus, a mixing chamber 44 is supplied with fuel and air from a fuel supply tube 41 and an air supply tube 42 respectively. The air is heated by a heater 43 before being supplied to the mixing chamber 44, so that a catalyst 45 in a first stage is heated. When the catalyst 45 in the first stage reaches a catalytic activation temperature, the air-fuel mixture is supplied through an air-fuel mixture supply tube 46, thereby heating a catalyst 47 in a second stage. When the catalyst 47 in the second stage reaches a catalytic activation temperature, the air-fuel mixture is supplied through an air-fuel mixture supply tube 48, thereby heating a catalyst 49 in a third stage. When the catalyst 49 in the third stage reaches a catalytic activation temperature, catalytic combustion is started. As a result, combustion gas 50 is dissipated outside to heat an object to be heated.
The conventional technologies described above pertain to a catalytic combustion element wherein a catalyst is arranged inside a combustor. In case of a catalytic combustion system having a catalyst carried on the front surface of a combustor, when combustion occurs on the front surface of the combustor, flames generated remain as they are without being blown off. It is essential for this type of combustor to hold the flames in position for combustion stability as well as easily controllable operations.
However, the conventionally employed catalytic combustor generally has the following disadvantages. First, the combustor is mostly made of a material such as cordierite or alumina so that the catalyst can be easily carried on the front surface thereof. These materials, however, have such a low thermal conductivity that the heat is liable to be retained in the combustor during combustion. In other words, a substantially adiabatic state occurs. According to FIG. 11, which illustrates the relation between excess air and combustion temperature, it may be understood that when the excess air factor is unity, the highest heat efficiency as well as the highest temperature is obtained. The excess air factor represents the excessive air in the air-fuel mixture, and when the excess air factor is unity the stoichiometric air-fuel ratio is achieved. For example, if the excess air factor is 2, there is twice as much air as contained in the air-fuel mixture at the stoichiometric air-fuel ratio. Therefore, it is desirable for combustion to set the excess air factor within the range of 1 to 1.2. However, if a burner is made of a material having low thermal conductivity as in the conventional technology, the temperature of flames in the adiabatic thermal equilibrium state reaches an undesirably high temperature of up to 1900.degree. C. At this high temperature, the catalyst oxide is no longer resistant to heat, and a material such as cordierite is no longer resistant to thermal shock. Accordingly, although an excess air factor within the range of 1 to 1.2 is desirable in principle, in practice, it is unavoidable to use a relatively lean air-fuel mixture whose excess air factor is 3. Thus, a comparatively bulky air supply blower that is able to supply a great amount of air is required, which results in an increase in the overall dimensions of the apparatus as well as a decrease in heat efficiency.
If the combustor did not have a catalyst carried thereon, the above-mentioned problem would be solved. In this case, however, it would solely depend on the geometry of the combustor whether the flames generated in gas-phase combustion are held appropriately. Since the flames are held less effectively by the geometry of the combustor than by a catalyst, even a slight increase in fuel supply would cause difficulties in holding the flames. That is, the flames would tend to be separated from the combustor. Consequently, in order to hold the flames effectively, the amount of fuel supply cannot be increased beyond a certain limit. Thus, it is impossible to produce an output higher than a certain limit.
Furthermore, the conventionally employed catalytic combustor requires the temperature of a catalytic layer carrying the catalyst to be always maintained equal to or higher than the catalytic activation temperature. It is necessary, therefore, to heat the air-fuel mixture all the time, which leads to a significant increase in the cost of combustion.